Process for producing colored oxide coatings



United States Patent Ofitice 3,077,425 Patented Feb. 12, 1963 This invention relates to the production of dyed colored surfaces on articles, and although it has other utility, it is particularly useful in connection with the production of colored oxide coatings on aluminum articles.

Such coatings are conventionally produced by anodizing an aluminum article to form a porous anodic coating thereon comprising aluminum oxide, dyeing the coating with a dye and then sealing the dyed coating to render it non-absorptive by immersing the article in hot water, a step which converts the aluminum oxide to aluminum oxide monohydrate. However, colored coatings produced in this way, while oftentimes suitably permanent for indoor use, are generally subject to fading or bleaching on prolonged exposure to sunlight.

It has been found that fade-resistant colored coatings may be obtained by incorporating into the coatings a rare earth metal chosen from the group consisting of cerium, ytterbium, samarium, gadolinium, lanthanum, neodymium, europium, dysprosium, holmium, erbium, thulium, and mixtures of at least two of the same. :In accordance with certain features of the present invention, the rare earth metal is incorporated into the dyed coating after the anodizing operation in which an aluminum oxide is formed and after the dye has been applied, but either before the sealing operation or simultaneously therewith, by treating the dyed coating with a solution of the salt of one or more of the rare earth metals referred to above and more specifically with an aqueous solution of a Water soluble salt of such a metal. However, as far as certain aspects of the invention are concerned, the rare earth metal or its ion may be incorporated into the dye, either as, an additive thereto or as an integral part thereof, before the application of the dye to the aluminum oxide coating or may be incorporated into the anodizing electrolytic bath for deposit in and on the oxide coating being formed, before the application of the dye to said coating.

Colored coatings in aluminum articles having a rare earth metal or its ion incorporated therein, have been found in accordance with the present invention to be fade-resistant for all practical purposes, as determined by accelerated fading tests under conditions in which the color of conventional coating was completely bleached.

A preferred class of rare earth metal salts comprises the group consisting of cerium salts, ytterbium salts, and a mixture of Samarium and gadolinium salts. Suitable salts in this group include cerous chloride, cerous sulfate, ceric sulfate, cerous nitrate, ceric ammonium nitrate, ceric ammonium sulfate, cerous acetate, cerous bromate, cerous bromide, cerous iodide, ytterbium sulfate, ytterbium chloride, ytterbium acetate, ytterbium perchlorate, mixture of samarium and gadolinium chlorides, mixture of Samarium and gadolinium nitrates, mixture of Samarium and gadolinium acetates, mixture of samarium and gadolinium sulfates, etc.

Cerium salts are preferred. They may not only be used per se, and in any degree of purity, but also in the form of mixtures with salts of other rare earth metals. Such mixtures, which are commercially available, are derived from monazite ore and have a composition very much like that in the ore. For example, a mixture of rare earth metal chlorides available may have an approximate composition as follows, weight basis, the rare earth metal being expressed as oxide:

Percent Cerium oxide- 17-47 Lanthanum oxide 9-24 Praseodymium oxide 2-5 Neodymium oxide 6l7 Samarium oxide 1-3 Gadolinium oxide l-2 Yttrium oxide 0.2 Other rare earth oxides 0.8

Of course, no simple chemical formula can be written for the mixture, but its composition approximates RCl .6H O

where R=rare earth metals and may be considered to act essentially as a salt of one metal, since the properties of the rare earth metals are so nearly alike, but act more specifically as a salt of cerium, since the cerium salt is the major rare earth metal salt constituent of the mixture.

Other mixtures similar to the foregoing may comprise the sulfates, nitrates, acetates, etc. of the constituent rare earth metals. Another suitable mixture is one referred to as didymium salts, these being derived from monazite ore, and the term didymium referring to the mixture obtained after removal of thorium and most of the cerium from the mixture of rare earths originally present in the ore. The didymium salts may have the following approximate composition, weight basis, the metal being expressed as oxide: 0.12% cerium oxide, 45-47% lanthanum oxide, 3233% neodymium oxide, 9-10% praseodymium oxide, S6% samarium oxide, 34% gadolinium oxide, 0.4% yttrium oxide, and 12% other rare earth oxides.

Dyes for coloring oxide coatings on aluminum include dyes from classes such as acid dyes, mordant dyes, direct dyes, solvent dyes, and natural dyes. The preferred dyes, however, are the metallized dyes, comprising metal complexes obtained by treating an isolated dye with an aque ous solution of a metal salt, the treatment usually involving heating a mixture of the dye and solution over a period of time, and recovering the product. The dye may be from any of the foregoing classes. Chromium complexes are preferred, being usually formed by heating the dye with a water soluble salt like chromium formate, chromium fluoride, chromium sulfate, etc. Other metal complexes are those of copper, cobalt, iron, etc. Some examples of metallized dyes are metallized acid dyes such as C.I. Acid Yellow 98, 99, and 101, these designations referring to the listing in the Colour Index. Other examples are C.l. Acid Orange 26, 72, 74, and 76; Cl. Acid Red 179, 180, 183, 184, 188, 198, and 212; CI. Acid Violet 55 and 58; Cl. Acid Green 12; CI. Acid Black 52 and S5. Metallized direct dyes are suitable, such as C.I. Direct Violet 46. A metallized solvent dye is Cl. Solvent Red 8. Metallized natural dyes are useful.

Another preferred class of dyes, related in chemical constitution to the metal complex dyes, are the mordant dyes, which are used with a solution of a soluble salt of a metal like chromium, aluminum, iron, tin or copper. Usually the metal hydroxide or oxide is precipitated and deposited upon the oxide or anodic coating, and this precipitate or mordant is subsequently united with the dye in situ to form an insoluble color lake, although the dye may be applied at the same time or even previous to the mordant. As indicated, the metal-dye product is formed in situ, whereas in the case of the metallized dyes the metal-dye product, or metal complex, is preformed. Some illustrative mordant dyes are C.I. Mordant Yellow 1, 3, 23, and 34; CI. Mordant Orange 1, 39, 40,

3 and 41; Cl. Mordant Red 5, 7, 17, and 81 to 84; CI. Mordant Violet 59 to 62; C.I. Mordant Blue 13, 31, and 69 to 71; Cl. Mordant Green 50 and 51; (3.1. Mordant Brown 24, 29, 30, 57, and 86 to 89; Cl. Mordant Black 9 and 75 to 77.

Natural dyes such as logwood extract also form lakes with metal compounds of the kind noted.

Other direct dyes are suitable, a preferred class being those designed for aftertreatment on the coating with salts of metals capable of forming coordination compounds, especialiy salts of copper and chromium such as copper sulfate, sodium bichromate, potassium bichromate, chromium fluoride, etc. These salts form metal complexes with the dye, and as in the case of the mordant dyes, the metal-dye product is formed in situ. Preferred specific dyes are Cl. Direct Red 79; C1. Direct Blue 71 and 79; C.I. Direct Black 19 and 51. Other examples of aftertreated dyes are C.I. Direct Yellow 8, 17, and 33; Cl. Direct Orange 6, 18, 39, and 56; CI. Direct Red 1, 83, and 99; CI. Direct Violet 9, 47, and 48; CI. Direct Blue, 1, 8, 15, and 22; C.l. Direct Green 1 and 22; Q1. Direct Brown 1, 2, 6, and 9.

Other useful direct dyes include Cl. Direct Yellow 35 and 50; Cl. Direct Violet 46.

Other suitable acid dyes are C.I. Acid Yellow 17, 23, 25, and 39; Cl. Acid Orange 40; Cl. Acid Red 14, 37, 73, 80, 82, 83, and 106; CI. Acid Violet 9, 17, 34, and 43; CI. Acid Blue 3, 15, 25, 35, 41, and 45; Cl. Acid Green 1, 20, and 25; Cl. Acid Black 1 and 2.

Still other dyes are solvent dyes like C.I. Solvent Yellow 19; CI. Solvent Red 35 and 71; Cl. Solvent Violet 2; Cl. Solvent Blue 25.

Chemically, the foregoing dyes belong to various classes, such as: azo, including monoazo, diazo, and triazo, triphenylmethane, xanthene, anthraquinone, oxazine, nitroso, and phthaloeyanine.

Referring to the metallized dyes involving a preformed metal complex, the mordant dyes involving formation in situ of a color lake, and the aftertreated dyes involving metal complexes formed in situ, it may be noted that in each case a metal-dye compound is formed involving the dye component and the metal or metal-containing component.

The article that is coated may be aluminum of any degree of purity, or a high aluminum alloy containing 50% or more aluminum. It will be understood that the term aluminum is intended to cover not only the metal but also any alloys in which it is the predominant constituent.

Except as indicated, the anodizing, dyeing and sealing steps are conventional, but a brief description of them follows.

In the anodizing step the aluminum article in the anode in an acid electrolyte. The electrolyte may comprise a 3% aqueous solution of chromic acid; or a 1 to 70%, preferably to aqueous solution of sulfuric acid; or a 3 to 10% aqueous solution of oxalic acid; or a mixture of aqueous sulfuric acid and aqueous oxalic acid. Conventional anodizing conditions are employed.

The coating of oxide may also be formed on the surface of the aluminum article by immersion in a suitable treating solution, generally alkaline, wherein the coating is formed by chemical reaction and without applying external electric current. In general, the coating may be formed by any suitable process, but it is preferably produced by anodic oxidation, especially in an aqueous solution of the kind noted, using externally applied electric current.

In dyeing, aqueous solutions of dyestuffs are employed, the concentration usually being 0.1 to 5% by weight for deeper shades and as low as 0.005% for pale shades. The dyestulf concentration is kept constant, dyestuii being added as it is absorbed, or lost by drag out, etc. The pH of the solution is usually 3 to 8, depending on the dyestuii. Dyeing may be carried out at room temperatures or higher, say F. to the boiling point. Application of the dye may be done in any convenient way, suitably by immersion for a time which may be less than one minute and up to 30 minutes.

In accordance with one specific embodiment of the present invention, the treatment with the rare earth metal salt solution is carried out simultaneously with the sealing step, and for that purpose, the article from the dye bath, after rinsing, is immersed preferably in hot water to which there has been added the Water soluble rare earth metal salt. The concentration of the salt may be up to the saturation concentration at the temperature of sealing. Precipitation of salt is preferably avoided. The preferred concentration may be between 20% by weight of the solution up to saturation. The usual temperature of the sealing solution is 160 to 212 F., and the time of immersion may be 5 to 40 minutes. If desired, sealing may be carried out with aqueous solutions having lower salt concentrations, for example, as low as 0.1% by weight, in which case the sealing temperature is lower, on the order of to 170 F., and the sealing time is longer, extending from less than one hour to one or more hours.

It is also feasible to treat the dyed coating with the salt solution in a separate step and to follow such step with sealing. In this case, the salt treating step may be carried out by applying to the coatfng an aqueous salt solution having 0.1% by weight of salt up to the saturation concentration at a temperature of about to 212 F. for a time of 5 to 40 minutes, after which the coating is subjected to a conventional sealing step with hot water.

Lower temperatures may be employed to incorporate the rare earth metals in the dyed coating than those set forth, but the treatment for that purpose would require a longer period.

The solution of rare earth metal salts, whether applied prior to the sealing operation or simultaneously therewith, should be maintained at a pH of 4 to 8 and preferably at a pH of about 6 /5 with suitable buffering agents, such as acetic acid.

The invention may be illustrated by the following examples.

Example I A number of pieces of strip aluminum, each 6" x 4;" x 0.03", were anodically coated in a conventional aqueous sulphuric acid anodizing oath at a potential of 18 volts, a temperature of 74 F, and an immersion time of 17 minutes. Each pIece was then rinsed in cold water and dyedin a 0.2% aqueous solution of Vitrolan Red GRE (Cl. Acid Red 183; Ci. 18,800), a monoazo metallized dye comprising a chromium complex, at a temperature of 150 F. for about 3 to 5 minutes. The pieces were rinsed with cold Water and then immersed in a sealing bath prepared by mixing one part by weight of rare earth metal chloride and two parts by weight of water. In terms of the metal oxide content, the rare earth metal chloride comprised 21.1% cerium oxide, 10.5% lanthanum oxide, 7.5% neodymium oxide, 2.2% praseodymium oxide, 1.3% samarium oxide, 0.9% gadolinium oxide, 0.1% yttrium oxide, and 0.4% other rare earth oxides, weight basis. The bath was maintained at about 210 F. and the pieces were immersed for about 10 minutes, after which they were dried, suitably masked off, and placed in a conventional fadometer for accelerated testing of the fastness of the color to light. At the same time, there were placed in the fadometer several control pieces of strip aluminum which had been treated as described except that the sealing bath did not contain rare earth metal chloride but consisted of water having dissolved therein a conventional sealing agent comprising nickel and cobalt acetates. The strips were examined at intervals of 40 hours. After the first 40 hours, the control strips exhibited almost complete fading. The strips treated according to the invention showed no perceptible fadlng after 500 hours, at which time the tests were discontinued. A qualitative spectrographic analysis of the dyed coating indicated the presence of a rare earth metal and especially cerium in the dyed oxide coating.

Example 2 Example 1 was repeated except that the dye was Vitrolan Orange R (0.1. Acid Orange 76; CI. 18,870), a monazo metallized dye comprising a chomium complex. After testing in the fadometer for 100 hours, the strips treated by the invention showed no fading, while control strips were substantially faded; after 200 hours the present strips were very very slightly faded, and the control strips almost completely faded; after 500 hours the present strips showed slight fading, and the controls were completely faded.

Example 3 This example was carried out as described in Example 1 except that Pyrazol Fast Blue BS (Cl. Direct Blue 71; Cl. 34,140), a trisazo dye, was used in the dyebath. After 100 hours in the fadometer the present strips were very slightly faded, after 200 hours the fading was slight, and after 500 hours it was still slight although more perceptible than that at the 200-hour stage. Control strips at each stage exhibited, respectively, substantial fading, almost complete fading, and complete fading.

Example 4 The results were similar to those indicated above in connection with Examples 1, 2 and 3 and the qualitative spectrographic analysis indicated essentially the presence of cerium in the dyed coating.

Aluminum articles having colored coatings as described are useful in decorative applications and in installations 6 where corrosion and/ or abrasion resistance was required.

In the light of the foregoing description, the following is claimed:

1. A metal article having a dyed, sealed, oxide coating thereon, said coating containing cerium to improve the light fastness of the dye.

2. An article as described in claim 1, wherein said article is aluminum.

3. In a method of producing a dyed surface on a metal whose oxide is capable of being dyed, the improvement comprising contacting a metal oxide coating on said metal with a solution of cerium salt to retain in said coating cerium ions, and maintaining said cerium ions in said coating in the presence of a dye coloring said coating to improve the light fastness of the dye.

4. The improvement as described in claim 3, wherein the metal is aluminum.

5. In a method of producing a dyed surface on a metal whose oxide is capable of being dyed, the improvement comprising contacting a dyed metal oxide coating on said metal with an aqueous solution of a water soluble salt of cerium at sufiiciently high temperature to seal said coating and at the same time to cause said coating to pick up and retain cerium ions from said solution to improve the light fastness of the dye.

6. In a method of producing a dyed surface on a metal whose oxide is capable of being dyed, the improvement comprising contacting a dyed metal oxide coating on said metal with a salt solution of cerium to cause said coating to pick up and retain cerium ions from said solution to improve the light fastness of the dye, and subsequently contacting the dyed metal oxide coating with the cerium ions retained therein with water at sufiiciently high temperature to seal the surface of said coating.

References Cited in the file of this patent FOREIGN PATENTS Great Britain Feb. 16, 1933 OTHER REFERENCES 

1. A METAL ARTICLE HAVING A DYED, SEALED, OXIDE COATING THEREON, SAID COATING CONTAINING CERIUM TO IMPROVE THE LIGHT FASTNESS OF THE DYE. 